1. Field of the Invention
The present invention relates to a rider-controlled working machine having a four-wheel steering system.
2. Description of Relevant Art
Rider-controlled working machines such as agricultural working machines or power-driven carts are required to turn with a small radius and hence make small sharp turns. To meet this requirement, it has been proposed to steer the front and rear wheels in opposite directions at the same time as disclosed in Japanese Utility Model Publication No. 29-4015, U.S. Pat. Nos. 1,028,866 and 3,596,730, and Swiss Pat. No. 216,854, for example.
The conventional four-wheel steering systems have linear characteristics such that the rear wheels are steered in the direction opposite to that in which the front wheels are steered even when the steering wheel is turned through a small angle. Since the steered wheels respond sharply to a slight turn of the steering wheel while the vehicle is running along a straight path, it has been difficult to keep the vehicle running straight. The prior rider-controlled agricultural working machines with four-wheel steering systems have therefore failed to cultivate a field along a straight path while pulling over at the edge of a ridge on the field being cultivated or at the step between a cultivated area and an uncultivated area.
Another problem associated with conventional rider-controlled working machines with four-wheel steering system occurs when an earth working unit such as an agricultural rotary working unit is coupled to the rear end of the working machine. When the front and rear wheel of the working machine with such an earth working unit coupled thereto is steered in opposite directions by a small turn of the steering wheel while the working machine is moving along a substantially straight path, the center of turning movement of the working machine is positioned between extensions of the axles of the front and rear wheels in the neutral steering positions. The radius of the turn of the rotary working unit at this time is larger than the radius of the turn of the working machine, with the result that the rotary working unit tends to accumulate soil on one side.
The present invention has been made in an effort to solve the above problems of the conventional rider-controlled working machines with four-wheel steering systems.